The present invention relates to a TV camera provided with a solid image pick-up element, and more particularly to such a TV camera that has an improved solution.
As a routine result, when sampling a video signal sent from a solid image pick-up element, the TV camera provided with a solid camera element such as a CCD (Charge Coupled Device) brings about generation of an aliasing distortion (hereinafter simply referred to as "aliasing"). If the TV camera does not use so high a sampling frequency, the aliasing is intruded into a video band, so that a moire phenomenon (hereinafter simply referred to as "moire") may take place in the reproduced image. This results in degrading the image quality. FIG. 1 shows a frequency characteristic of relative gains of an aliasing and a video signal. In FIG. 1, fs denotes a sampling frequency.
As a method for reducing the aliasing indicated by a broken line and an alternate long and short dash line of FIG. 1, conventionally, the method called "Shifting of Space Pixel" has been known and published in U.S. Pat. No. 4,069,501 corresponding to JP-B-55-19553. This method uses a matrix operation function executed in the TV camera. As shown in FIG. 2, this method is arranged so that the pixels located on a light-receiving portion of a G (Green) channel CCD are shifted in the horizontal scan direction by a half of a horizontal pixel interval Px between the adjacent pixels with respect to the pixels located on the light-receiving portions of a R (Red) channel and a B (Blue) channel CCDs, respectively. This location of each CCD results in making the video signals of the R and B channels out of phase by 180 degrees with respect to the video signal of the G channel, thereby making the aliasings of those channels out of phase by 180 degrees as indicated by the dotted line and the alternate long and short dash line of FIG. 1. Herein, the resulting luminance signal Y, which is obtained by performing the matrix operation about the signals of the G, R and B channels, is represented by the following expression, for example: EQU Y=0.59G+0.30R+0.11B
wherein G, R and B denote the signal levels of the G, R and B channels, respectively. The white level ratio, that is, the ratio of white level (white reference voltage level) of the G, R, B channels in 1:1:1.
In this luminance signal Y, therefore, the aliasing component of the G channel (indicated by the dotted waveform of FIG. 1) is partially offset with the aliasing components of the R and B channels both in opposite phase to that of the G channel (indicated by the alternate long and short dash line of FIG. 1), which results in reducing the aliasing components. Actually, however, all of the aliasing components are not offset. About 18% of the aliasing (indicated by a two-dot chain line of FIG. 1), which is derived from the operation of 0.59=(0.30+0.11)=0.18, is left in the luminance signal (Y). In the frequency band around 3/4 of the sampling frequency fs, that is, the frequency around 0.75 fs, hence, the substantially same aliasing components as the video signal components are left. In the higher frequency band than 0.75 fs, more aliasing components than the video signal components are able to be left. These aliasing components surpass the allowable level in light of the S/N ratio and thus are an obstacle to improving the resolution of the image.
Then, a further improved method was developed for improving a resolution of a multi-panel, for example, three-panel type CCD color TV camera with the "shifting of space pixels" method. This method is described in JP-B-61-7073 and shown in FIGS. 3 to 5 of the drawings appended to this specification. FIGS. 3 to 5 show the corresponding prior arts. The prior art shown in FIG. 3 (called the first prior art) is arranged to have three solid image pick-up elements 101R, 101G and 101B, three sample and hold circuits (SHs) 102R, 102G and 102B, three low-pass filters (LPFs) 103R, 103G and 103B, an adder 105, an attenuator (ATT) 106, a high-pass filter (HPF) 107, and three synthesizers 108R, 108G and 108B. In operation, the solid image pick-up elements 101R, 101G and 101B supplies the corresponding video signals. These video signals are sampled and held in the sample and hold circuits 102R, 102G and 102B, respectively. Then, these sampled and held signals S.sub.RH, S.sub.GH and S.sub.BH pass through the low-pass filters 103R, 103G and 103B from which the low-pass components R.sub.L, G.sub.L and B.sub.L of the primary color signals are derived. At once, these signals S.sub.RH, S.sub.GH and S.sub.BH are added in the adder 105. The added output is sent to the attenuator 106 in which the signal level is reduced by a factor of 3. The level-reduced signal is sent to the high-pass filter 107 from which a high-pass component signal W.sub.H is derived. The high-pass signal W.sub.H is added to the low-pass components R.sub.L, G.sub.L and B.sub.L through the effect of the corresponding synthesizers 108R, 108G and 108B. The resulting signals are made to be the high-resolution primary color signals R, G and B.
Next, the prior art shown in FIG. 4 (called the second prior art) will be described. The second prior art is arranged to have three solid image pick-up elements 101R, 101G and 101B, sample and hold circuits 102R, 102G and 102B, low-pass filters 103R, 103G and 103B, a sampling circuit 110A, a holding circuit 110B, a high-pass filter 107, and synthesizers 108R, 108G and 108B. In operation, the three solid image pick-up elements 101R, 101G and 101B supply the video output signals S.sub.R, S.sub.G and S.sub.B These signals S.sub.R, S.sub.G and S.sub.B are sampled and held in the corresponding sample and hold circuits 102R, 102G and 102B. The sampled and held signals are sent to the low-pass filters 103R, 103G and 103B through which the low-pass components R.sub.L, G.sub.L and B.sub.L of the primary color signals are derived. At once, the video signals S.sub.R, S.sub.G and S.sub.B are sent to the sampling circuit 110A in which the signals are changed in turn at a one-third period based on a sampling pulse Ps. The signals S.sub.R, S.sub.G and S.sub.B are supplied to the holding circuit 110B by turns. The signal outputted from the circuit 110B is sent to the high-pass filter 107 through which derived is a high-pass component W.sub.H with an improved frequency characteristic. This high-pass component W.sub.H and the low-pass components R.sub.L, G.sub.L and B.sub.L are added to each other at the corresponding synthesizers 108R, 108G and 108B respectively. The added signals are made to be the high-resolution primary color signals R, G and B.
Further, the prior art shown in FIG. 5 (called a third prior art) is arranged to have three solid image pick-up elements 101R, 101G and 101B, capacitors 112R, 112G and 112B, a sample and hold circuit (SH) 110, a high-pass filter 107, a trap circuit 114, sample and hold circuits 102R, 102G and 102B, low-pass filters 103R, 103G and 103B, and synthesizers 108R, 108G and 108B. In operation, the three solid image pick-up elements 101R, 101G and 101B supply the corresponding video signals to the corresponding capacitors 112R, 112G and 112B. These capacitors operate to cut dc components of these signals and send the dc-cut signals to the sample and hold circuit 110 in which the signals are changed by turns at a one-third period, based on the sampling pulse Ps. The sampled and held signal is sent to the high-pass filter 107 and then the trap circuit (TRAP) 114 for trapping a carrier frequency F.sub.C, from which a high-pass signal W.sub.H is derived. The high-pass signal W.sub.H is synthesized with the low-pass signals R.sub.L, G.sub.L and B.sub.L sent at the corresponding synthesizers 108R, 108G and 108B from the corresponding sample and hold circuits 102R, 102G and 102B. The synthesized signals are made to be the high-resolution primary color signals R, G and B.
The foregoing conventional solid image pick-up devices, that is, TV cameras, have the following shortcomings.
In the first prior art, though the three solid image pick-up elements are subject to the foregoing "shifting of spacial pixels", the high-pass component obtained by taking an adding means of the three video outputs through the adder 105 and the attenuator 106 contains the averaged video output, which has the reduced difference among the video signals by a one-third period. That is, the averaged video output lessens the effect of the "shifting of spacial pixels" and thus does not make so much contribution to enhancing the resolution. In other words, when the three video signals are simply added and averaged the sampling period becomes substantially longer, thereby suppressing the extension of the frequency band and lessening the effect of the "shifting of spacial pixels".
In the second and the third prior arts, as mentioned above, the output signals of the CCDs are sampled and held at a one-third period by the sample and hold circuit. The resulting high-pass component is synthesized with the low-pass component signals at a later stage of the sample and hold circuits 102R, 102G and 102B through the effect of the corresponding synthesizers 108R to 108B. This arrangement needs as many as three synthesizers. Further, if the signals with the high-pass component are digitized at a later stage of the synthesizers 108R to 108B, for example, re-sampled at the same frequency of the sampling frequency of the CCD, the sampling makes an aliasing of the high-pass component, which eliminates the effect of improving the resolution and produces more aliasing-based moires. Apparently, more moires make the quality of the video signal less. To eliminate the aliasing, for example, it is necessary to use three times as large a frequency as the sampling frequency of the CCD for digital processing the signals.